They are among the largest reptiles on earth whose dozens of razor-sharp, serrated teeth are so teeming with bacteria that one bite can prove lethal to a human; that's if the venom doesn't get you first.

But scientists have now discovered that the Komodo dragon is even more remarkable than they first thought. In what seems to be the reptilian equivalent of a "virgin birth", giant lizards at two zoos in the UK have laid eggs without having mated. Four of the eggs at London zoo have hatched; another eight at Chester zoo are due to hatch within weeks.

Self-fertilisation has never before been documented in the species. "We've ruled out any potential father," said Richard Gibson at the Zoological Society of London, who has been monitoring the progress of the fatherless baby lizards at London zoo.

"Not that a male could get in with a female without someone noticing, but just to make sure that everyone was completely convinced we DNA fingerprinted everybody and everything."

Kevin Buley, Chester zoo's curator of lower vertebrates and invertebrates, added: "We will be on the lookout for shepherds, wise men and an unusually bright star in the sky over Chester zoo."

Details of the births will be published today for the first time in Nature. A captive-bred female called Sungai laid a clutch of 22 eggs at London zoo in 2005; four babies hatched in March this year. Sungai last had contact with a male called Kimaan two and a half years previously. But genetic tests on the offspring ruled Kimaan and all the other males at London zoo out as potential fathers.

Now the focus has turned to the eggs of Flora, a female at Chester zoo. Flora has never had any contact with a male. She was brought to the zoo aged around one year and females do not reach sexual maturity until four or five.

Flora laid a clutch of 25 in May of which eight eggs still survive. Similar genetic detective work has ruled out the influence of any other male, today's report says. The tests suggest that instead of having two different sets of chromosomes from a mother and father, the offspring have two identical sets of chromosomes, both from their mother.

Komodo dragons typically grow up to three metres long and have long yellow tongues. They can sprint short distances at around 13 miles per hour and are good swimmers and climbers. There are fewer than 4,000 in the wild, scattered among the islands of the Indonesian archipelago.

The researchers think the eggs in Chester and London were produced by a process called parthenogenesis - meaning "virgin birth" in Greek - in which females produce offspring without fertilisation from a sperm.

"It's not very clear exactly what happens and we don't know whether it has occurred in the wild. But the fact that it has happened twice in such a short space of time with two unrelated females suggests that it is more common than we think," said Mr Gibson.

Parthenogenesis is common in invertebrates such as wasps, aphids and water fleas but it is rarely found in backboned animals. A handful of reptiles and fish can do it and it has also been found in turkeys.

The eggs at Chester zoo are around 10cm long, but when the baby dragons hatch they will be up to 45cm long, although around half of this is a slender tail. "They are an incredible feat of packaging," said Mr Gibson.

"They are not the easiest species to keep in captivity. They have exacting standards. You need very high-quality accommodation mimicking the high temperatures and high UV light that they would be exposed to in the wild."

In the wild, young lizards receive no care from either parent. They emerge from their eggs and dig their way out of the hole their mother buried them in. Then they head up into the trees where they feed on insects, birds' eggs, nestlings and smaller lizards. When they reach sexual maturity they start spending more time on the ground.

Going solo

Jesus: Was Mary a virgin "overshadowed" by the Holy Ghost or was the birth of the Messiah the most elaborate cover up for an affair in history? Scholars continue to argue over the meaning of the Hebrew word Almah in the book of Isaiah. It has been translated as both virgin and young woman.

Reptiles: There are a handful of reptiles known to reproduce by parthenogenesis. There are 15 species of New Mexico whiptail for example that have no males at all and reproduce exclusively that way. Strangely, females still act out courtship rituals with each other.

Aphids: Some of the most obvious animals capable of virgin birth are the greenfly and blackfly that attack garden plants. They produce large numbers of young quickly by parthenogenesis. In some species, the developing female inside the mother has a developing female inside her.